1. Field of the Invention
The present invention relates to a vehicle control system for stabilizing the behavior of a vehicle by controlling the braking force and the driving force applied to left and right wheels to generate a yaw moment, when the actual motion state of the vehicle deviates from a reference motion state.
2. Description of the Prior Art
There is a prior art vehicle control system which is designed, so that when a vehicle is in an unstable traveling state, whereby it is impossible to control the direction of movement of the vehicle according to a driver's will, a yaw moment for recovering control of the vehicle is generated to maintain a stable state by individually controlling the braking force and the driving force applied to left and right wheels.
With such a vehicle control system, a reference lateral slip angle and a reference yaw rate presumed to be generated by the vehicle in its stable state, are determined based on the vehicle speed and the steering operation conducted by a driver, and an actual lateral slip angle and an actual yaw rate actually generated in the vehicle are compared with the reference lateral slip angle and the reference yaw rate, respectively. To converge the deviations resulting from the comparison to 0 (zero), the braking force and the driving force applied to the left and right wheels are controlled individually in a feedback manner. Therefore, the following problems may occur.
If the vehicle is brought into a state in which the actual lateral slip angle and the actual yaw rate are smaller than the reference lateral slip angle and the reference yaw rate (i.e., into an under-steered state, for example), when the vehicle is traveling on a road near an exit of a curve, the conventional control allows a braking force to be applied to the inner wheels as viewed during turning of the vehicle, to generate a yaw moment in a direction to assist in the turning of the vehicle in order to eliminate the under-steered state. In the vehicle traveling near the curve exit where the curved road is changed to a straight road, however, the yaw moment in the direction to assist in the turning of the vehicle is generated excessively by the conventional control and hence, there is a possibility that a sense of incompatibility is provided to the driver. Such problem arises, because the conventional control is carried out based on a deviation between a reference direction of movement of the vehicle and an actual direction of movement of the vehicle, and the actual shape of the road is not taken in consideration. Namely, when the vehicle is traveling on the road near the exit of the curve, the control is carried out based on the traveling state on the current curved road and no consideration is given to the state in which the shape of the road ahead of the vehicle is such that a curved road is changed to a straight road. Therefore, the appropriate control adapted for the actual shape of the road cannot be conducted in some cases.